B757-200 First Officer reported an unauthorized vehicle on the taxi way caused the flight crew slow and stop to avoid a collision. After the unauthorized vehicle drove under the left wing and continued on the taxi way. The encounter was reported to Ground; and the aircraft continued to the gate.

Narrative

After landing on Runway XXR in ZZZ; we were given instructions to taxi to the Company Ramp from ZZZ Tower. Those instructions were 'A4; right turn A to park and stay with him'. I read the instructions back and the Captain acknowledged them. As we were turning onto A from A4; he called for the After Landing Checklist. After verifying Taxiway A was clear on the right; I proceeded to clean the aircraft up and complete the checklist. This required me to be heads down for a period of time in order to accomplish all items. As I was finishing the checklist; I noticed we had slowed almost to a stop. When I looked up I saw the flashing lights of an Ops vehicle ahead of us and then noticed a single light approaching the aircraft. The Captain stopped the aircraft short of Taxiway A5.The vehicle continued to head towards us. At this point we could make out that it was a white truck with markings on the side. The light was the single working headlight. The vehicle slowed briefly then sped under the left wing. The Ops vehicle following stopped short of Taxiway A. As this was happening; I called Tower to report we had stopped for a vehicle on the taxiway. Almost immediately after that; the Ops vehicle reported an unauthorized vehicle on A and they were trying to chase it down. Tower gave them instructions and we called back to verify that all vehicles were clear as the one had driven under the wing. Once we were told they were; we continued our taxi to the ramp without further incident. After Shutdown; the Captain notified the Dispatcher and Duty Officer to inform them of what happened. Though we did not feel like the vehicle made contact with the aircraft; the pre-flight for the following flight confirmed no damage to the aircraft.